The present invention relates to a technique for measuring the quality of an electronic assembly soldering process. Specifically, a test vehicle is provided which may be used in a surface insulation resistance (SIR) test to evaluate the quality of an electronic assembly soldering process.
The manufacture of electronic circuits includes mounting integrated circuit components onto a single printed circuit board of a module which may be used in a larger electronic system. The circuit board includes conductor patterns which terminate in a connection to either a component, or a connector for making external connections to the circuit board. The process of preparing the board is well known. which can involve a variety of soldering and cleaning techniques. The solder process typically involves the application of flux, solder, and heat to the solderable areas on the circuit board in order to form a soldered connection to components, followed by cleaning to remove unwanted soldering byproducts. The solderable areas include not only connection pads for components, but plated through via holes which extend to the surface of the printed circuit board. The via holes provide a connection between conductors on opposite sides of the printed circuit board.
The soldering techniques typically employed to make external connections to the circuit board include solder paste reflow, wave, fountain, thermode, laser, and hot gas soldering. Subsequent removal of post-soldering byproducts is usually accomplished using aqueous or solvent-based chemistries in a cleaning machine, or without cleaning if no-clean soldering materials are used.
The process of removing the soldering byproducts becomes more difficult as the density of circuit conductors on the circuit board increases, and as the profile or dimensions of the components on the board and the relevant spacing between the components and the board decreases. An incomplete removal of the soldering byproducts has been known to produce latent electrochemical migration or corrosion effects between conductive elements on the assembly. This is particularly acute when a mildly activated resin or organic acid based flux systems are used. The electrochemical migration or corrosion effects may result in latent product reliability problems, such as open circuits caused by corrosion or electrical shorts resulting from electrochemical migration.
One of the techniques which has been developed for evaluating the ability of an electronic assembly process to adequately remove soldering byproducts is known as the surface insulation resistance (SIR) testing technique. The SIR testing methodology has been developed with industry standard specifications controlled by the Electronic Industry Association (EIA) and the Institute for Interconnecting and Packaging Electronic Circuits (IPC). SIR testing employs the use of a special printed circuit board test vehicle which is processed through an electronic assembly manufacturing process. The test vehicle includes four wiring patterns of interleaved conductors which are not normally connected together. The wiring patterns comprise two electrical conductors having a width of 0.016 inches and a spacing of 0.020 inches between the conductors. The test vehicle is subjected to the soldering and cleaning process without any external components or a solder mask which is used in the majority of useable electronic assemblies. Thus, in some ways the standard SIR testing techniques are deficient since the process of removing soldering byproducts from a useable circuit board is effected by the component placement on the printed circuit board.
As new component technology reduces the profile of the various circuit packages which are attached to a printed circuit board, the spacing between the printed circuit board and the components gets smaller, presenting a challenge for removing soldering byproducts which may reside between components and circuit board. Whereas previous component bodies are spaced 0.01 inches from the board surface, facilitating the removal of soldering byproducts which enter the space, the newer components provide a spacing of 0.002 inches which increases the difficulty in removing soldering byproducts which may be trapped beneath the component. This in turn increases the risk for latent electrochemical migration or corrosion which cannot be tested using a test vehicle which does not account for the reduced spacing between the circuit board and components.
The additional improvements in reducing the size of electrical conductors also presents a greater difficulty in evaluating the potential for electrochemical migration or corrosion. Whereas the previous test vehicles used a conductor width of approximately 0.016 inches and conductor spacings of 0.020 inches, newer printed circuit designs allow for a conductor spacing of 0.006 inches versus the previous 0.020 inch conductor spacing.
Given the foregoing improvements in electronic assembly density, a better test vehicle is needed for evaluating the soldering and cleaning processes in this new high density environment.
It is an object of this invention to provide a SIR test vehicle for evaluating printed circuit manufacturing processes for electronic assemblies.
This and other objects of the invention are provided for by a printed circuit board which supports a plurality of interleaved circuit patterns. The interleaved circuit patterns have a pair of spaced apart conductors which terminate on a via hole on the circuit board. The via holes on the circuit board are connected by a conductor within the hole to a conductor on the underside of the circuit board. A replica component having the dimensions and lead configurations of an actual component is supported on the circuit board over at least one of the interleaved patterns. The test vehicle with the replica component is subjected to the manufacturing process through which actual circuit board assemblies are to be fabricated. The interleaved circuit patterns provide an ability to measure the effects of electrochemical migration or corrosion which occurs as a result of unwanted soldering byproducts being incompletely removed from the circuit board surface. The replica components are supported above the circuit board at distances which are contemplated for actual components which are to be used on circuit boards. The effects of soldering byproducts which remain on the electronic assembly, and particularly soldering byproducts which reside between the replica component and the circuit board, may be determined by measuring the resistance between the interleaved conductors of the circuit pattern.
In a preferred embodiment of the invention, not only is the resistance between the interleaved circuit patterns measured as an indicator of the manufacturing cleaning process effectiveness, but the replica circuit component connections to the circuit board are divided into two groups, and resistance measurements are made between groups of connections as an indication of the potential for electrochemical migration or corrosion effects.